1. Field of the Invention
This invention relates to code division multiple access communication systems, code division multiple access transceivers, and to methods for operating them.
2. Discussion of the Related Art
Code Division Multiple Access (CDMA) has become one of the major technologies for digital wireless communications in the U.S. and worldwide. Growing demand for the service provided by CDMA has created a need for expanded data rates and higher system capacity. Working against the expansion of system capacity is the problem that some users may receive an inferior signal because of multipath fading that is a property of the particular channel in use. However, signal quality is improved by the use of diversity reception, in which multipath fading of a particular signal is overcome by receiving and combining two or more reflections of the signal. This technique works when two or more reflections are sufficiently separated in time so that they can be resolved, that is, distinguished and separated at the receiver. This desirable situation depends on the presence of radio reflections generated by the environment.
However, for some users in a CDMA system no set of resolvable signals (xe2x80x9cno resolvable multipathxe2x80x9d) will exist. For example, this will occur at a particular receiver if the delay spread among the received reflections of the signal is less than one xe2x80x98chipxe2x80x99 duration. In this art, a xe2x80x98chipxe2x80x99 is a characteristic duration that is approximately equal to the inverse of the system bandwidth. When the delay spread among received reflections is less than one chip, the receiver cannot adequately distinguish and separate the signal reflections and therefore can provide no reduction in signal fading. This adverse situation can arise in both indoor and outdoor cellular systems. While it has been proposed to increase transmitter power to avoid such situations, such a tactic greatly reduces system capacity and increases interference. Alternatively, it is known that transmission diversity can be used, but in such proposed systems the cost in terms of reducing system capacity and increasing interference is significant.
According to the present invention, in a code division multiple access communication system, a first transceiver has on its transmitter side a plurality of antennas disposed to provide transmission using a plurality of paths; and a second transceiver has on its receiver side a xe2x80x9crakexe2x80x9d arrangement for processing a multiplicity of received signal versions. Relatively few, ordinarily, one, of the plurality of antennas in the first transceiver is used normally. When, however, the first transceiver obtains an indication that the receiver of the second transceiver is not receiving a sufficient number of resolvable signal versions, the first transceiver is switched to use more of the plurality of antennas. Thus, multipath fading is overcome and the capacity of the system is favorably affected, in that it is not necessary to increase total transmitted power and the diversity order is not increased unnecessarily for those users already obtaining adequate diversity signals through the radio-reflective multipath environment.
A first implementation of the invention feeds back a signal from the second transceiver to provide the indication that resolvable multipath does not exist for the channel in use at its receiver side. The transmitter side of the first transceiver is then switched to use the increased number of antennas and thus provide transmission diversity of signals transmitted to the second transceiver.
A second implementation of the invention provides that the first transceiver detects at its receiver side that resolvable multipath does not exist at its receiver side for the channel in use for signals from the second transceiver. The first transceiver switches its own transmitter side to provide transmission diversity of signals transmitted to the second transceiver, Under the detected condition, it is at least likely that resolvable a multipath does not exist at the receiver side of the second transceiver. Since in some systems, such as time-division duplexing (TDD) systems, the number of diversity paths on the up and down links will be identical, in the same period of time in which the first transceiver is switching, the second transceiver will have made the same adaptation. Thus, feedback is not necessary, as the requisite number of paths in both directions on the channel can be determined by each transceiver independently.
According to a further implementation of the invention, adaptive transmission diversity is provided by employing additional spreading codes, rather than delays, at the additional antennas on the transmitter side of each transceiver when no resolvable multipath exists.
According to a first aspect of the invention, a transceiver for code division multiple access communication has on its transmitter side a plurality of antennas disposed to provide transmission using a plurality of paths and on its receiver side a demodulator and demultiplexer for signals received from a remote transceiver. The transceiver has an arrangement to use relatively few, e.g., one, of the plurality of antennas normally. When, however, there is received an indication of no resolvable multipath, the arrangement switches the transceiver to use more of the plurality of antennas.
According to a second aspect of the invention, a transceiver for code division multiple access communication has on its transmitter side a signal splitter and modulator for data signals and has on its receiver side both a rake arrangement for attempting to separate a plurality of received signal versions from a remote transceiver and a searcher for searching for a plurality of resolvable signal versions. The searcher is connected from the rake arrangement to the signal splitter on the transmitter side to provide an indication signal for signal splitting whenever the searcher does not find a plurality of resolvable signal versions.
Various features of the invention reside in the particular arrangements for providing an indication signal and/or switching of antennas on a transceiver transmitter side and in the methods of operation, as will become clearer hereinafter.